The invention concerns a process for the cylinder-selective leakage testing of the combustion chambers in combustion engines during engine operation, but with the combustion process being suppressed, as known from DE 197 34 680 A1.
Engine operation with a suppressed combustion process occurs for instance when the combustion engine is started by means of an electric starter without any fuel being supplied and without any external ignition, or when a motor vehicle rolls on a downhill gradient whilst the engine transmission is engaged and the overrun fuel cut-off is activated.
A device for evaluating the compression of a multi-cylinder combustion engine by means of a process of the general type mentioned above is known for example from DE 43 37 720 A1. With this process, a signal is evaluated which is proportional to the starter current during combustion engine start-up. Here, the significant increase of the required starter current for gas mixture compression during the compression stroke of a cylinder is evaluated. If a cylinder shows a leakage of any kind, this can be detected by measuring the starter current that should normally be applied.
This method of leakage testing the combustion chambers is meaningful for combustion engines with electric starters which are not equipped with the means for detecting the crankshaft rotation angle and determining the current crankshaft speed.
The disadvantage of this process is that the leakage testing of the combustion chambers can be effected only on compression of the gas mixture during the compression stroke of the combustion engine cylinders. Moreover, the components required for measuring the starter current can be used for leakage testing only; therefore, this method of leakage testing is very complicated and expensive.
From the general basis established by DE 197 34 680 Al, there is known a process for the cylinder-selective leakage. testing of the combustion chambers in combustion engines by means of detecting the crankshaft rotation angle and determining the current crankshaft speed during motor operation with a suppressed combustion process. With this process, characteristic values are derived and obtained, during the compression and/or combustion strokes of the combustion engine cylinders, from the characteristic curves of the current crankshaft speeds. These characteristic values are correlated to the actual compression pressure in the cylinder combustion chambers, thus providing for leakage defects in the combustion chambers of the combustion engine to be detected cylinder-selectively.
The disadvantage of this process is that the statement on the cylinder-selective compression pressure can be made only by comparing the cylinders against each other. In so doing, this statement will be falsified by unavoidable cross-influencing of all characteristic values among the combustion chambers. Such cross-influencing occurs for example due to the fact that in the event of a combustion chamber having a leakage the rotation of the crankshaft is retarded less so that the combustion chamber which subsequently is to be compressed is actually compressed at a higher crankshaft speed. This makes it impossible to assess the cause of a combustion chamber leakage. In addition, this process cannot be used for combustion engines with just a single combustion chamber.
It is an object of the invention to provide an improved process for the leakage testing of the combustion chambers in combustion engines, by means of which process the compression pressure within the combustion chambers can be determined directly, with any cross-influences among the combustion chambers being compensated, and which process can also be used for combustion engines having just a single combustion chamber.
The above object has been achieved according to the invention in a process for the cylinder-selective leakage testing of at least one combustion chamber of a combustion engine. In the inventive process, current engine shaft speeds are determined from the engine shaft rotation angles for each cylinder of the combustion engine. Here, engine shaft rotation is subdivided into a limited number of angle corresponding to the number of angle segments to be determined. The crankshaft speed will then change only from one angle segment to the next, i.e. within an angle segment the engine shaft speed will be assumed to be constant.
Next, engine shaft speed differentials will be determined for each cylinder from the course of engine shaft speeds. For this, it is advantageous to use the increase in engine shaft speed following movement of a piston through the top dead center of a cylinder.
These engine shaft speed differentials are each allocated a characteristic value which is preferably formed respectively as a percentage share of the engine shaft speed differential of a cylinder relative to the highest determined engine shaft speed differential of all cylinders.
These characteristic values are allocated the value of the compression pressure, with the maximum compression pressure corresponding to the nominal compression ratio under normal conditions.
The compression pressures determined for each cylinder of the combustion engine are compared with a specified threshold value, if necessary with a cylinder-specifically specified threshold value, with a fault signal being output if the actual value falls below the threshold value.
Before leakage testing of the combustion chambers in the combustion engine is carried out, it is intended that a test for compliance with the constraints prerequisite for any leakage testing is carried out. Such a constraint is the minimum engine shaft speed, for instance, from which the compression pressure can be determined reliably from an engine shaft speed differential.
The engine shaft speed differential of a cylinder is determined by determining the value of the maximum current engine shaft speed differential after the piston of the cylinder has moved through the top dead center position, minus the actual current engine shaft speed directly following the top dead center of the same cylinder of the combustion engine.
In order to improve evaluation precision, the engine shaft speed differentials of the cylinders are corrected relative to these mean engine shaft speeds of the rotation angle segments in accordance with characteristic curves/characteristic fields or an algorithm that can all be applied.
In addition, the cylinder-specific engine shaft speed differentials are corrected relative to the engine temperature by means of characteristic curves that can be applied.
In the case of combustion engines with multiple cylinders the engine shaft speed differential for the next cylinder in the ignition sequence is corrected in relation to the engine shaft speed differential of the last cylinder in the ignition sequence.
A further development of the invention provides for the compression pressures of the cylinders and/or the cylinder-selective engine shaft speed differentials and/or the characteristic values to be stored for comparative purposes after the combustion engine has been manufactured, repaired, or at any other intervals required.
Another further development of the invention provides for irregularities of the compression pressures of the combustion chambers in the combustion engine to be displayed e.g. on the instrument panel of a motor vehicle.
In order to condition the combustion engine it is provided that, before starting the detection of the current engine shaft speeds, the combustion engine runs through e.g. a working cycle corresponding to 720xc2x0 in crankshaft rotation.
For example, the crankshaft speed, the camshaft speed or the starter shaft speed can be used as engine shaft speeds.
A final development of the invention provides for the leakage test process to be used in combustion engines featuring just a single cylinder, and with the engine shaft speed differential being compared with a stored engine shaft speed differential.